Pyrogenically prepared silicon dioxide

ABSTRACT

Pyrogenically prepared silicon dioxide with the following physicochemical properties:  
     1. Average particle size (D 50  value)D 50 ≧150 nm (dynamic light scattering, 30 wt %)  
     2. Viscosity (5 rpm, 30 wt %)η≦100 m·Pas  
     3. Thixotropy of T 1 (η(5 rpm))/(η(50 rpm))≦2  
     4. BET surface area 30-60 m 2 /g  
     5. Compacted bulk=100-160 g/L  
     6. Original pH≦4.5  
     can be used for the preparation of dispersions and glass bodies.

[0001] The present invention relates to a pyrogenically prepared silicondioxide.

[0002] Pyrogenically prepared silicon dioxide or pyrogenically preparedsilica is known from Ullmann's Encyclopedia of Industrial Chemistry,4^(th) edition, Volume 21, page 464.

[0003] It is known to use pyrogenically prepared silicon dioxide in thepreparation of glass bodies (U.S. Pat. No. 5,207,814, EP 0 586 013 B1,EP 0 705 797 A2, EP 0 131 057 B1, U.S. Pat. Nos. 4,681,615, 4,801,318).Other uses for silica are numerous and are well known in the art.

[0004] The known pyrogenically prepared silicon dioxide has the drawbackof presenting an unsatisfactory viscosity behavior in the dispersionused for the preparation of glass.

[0005] Thus, an object of the present invention is to overcome theproblems relating to pyrogenically prepared silicon dioxide for use inthe production of glass.

SUMMARY OF THE INVENTION

[0006] The above and other objects of the present invention can beachieved by using a pyrogenically prepared silicon dioxide with thefollowing physicochemical properties:

[0007] 1. Average particle size (D₅₀ value) D_(50≧)150 nm (dynamic lightscattering, 30 wt %)

[0008] 2. Viscosity (5 rpm, 30 wt %) η≦100 m·Pas

[0009] 3. Thixotropy of T₁ (η(5 rpm))/(η(50 rpm))≦2

[0010] 4. BET surface area 30-60 m²/g

[0011] 5. Compacted bulk density SD=100-160 g/L

[0012] 6. Original pH≦4.5

[0013] Measurement methods

[0014] Particle Size

[0015] Measurement method used according to the invention: Photoncorrelation spectroscopy (PCS) is a dynamic light scattering processwhich allows the detection of particles in the range of approximately 5nm to 5 μm. From the measurement results one can also calculate aparticle size distribution in addition to the mean particle diameter.

[0016] Light source: 650 nm diode laser

[0017] Geometry: 180° homodyne scattering

[0018] Sample quantity: 2 mL

[0019] Calculation of the distribution according to the Mie theory

[0020] Procedure: 2 mL of dispersion (30 mol %) are introduced into ameasurement cuvette, the temperature sensor is inserted, and themeasurement is started. The measurement is carried out at roomtemperature.

[0021] Viscosity

[0022] Measurement method for determining viscosity: A programmablerheometer is available to examine complex flow behaviors, and it isequipped with standard rotation spindles.

[0023] Shearing rate: 5-100 rpm

[0024] Measurement temperature: room temperature (23° C.)

[0025] Dispersion concentration: 30 mol %

[0026] Procedure: 500 mL of dispersion are introduced into a 600-mLglass beaker and examined at room temperature (statistical determinationof the temperature via measurement sensors) at different shearing rates.

[0027] BET: based on DIN 66131

[0028] Compacted bulk density: based on DIN ISO 787/XI K 5101/18 (notsieved)

[0029] pH: based on DIN ISO 787/IX, ASTM D 1280, JIS K 5101/24.

[0030] The pyrogenically prepared silicon dioxide according to theinvention can be prepared by mixing a volatile silicon compound, suchas, for example, silicon tetrachloride or trichloromethylsilane with anoxygen-containing gas and hydrogen, and by burning this gas mixture in aflame.

[0031] The pyrogenically prepared silicon dioxide according to theinvention can be used advantageously for the preparation of dispersionsin aqueous and/or nonaqueous solvents.

[0032] An additional feature of the invention is a dispersion in aqueousand/or nonaqueous solvents, which dispersion contains the pyrogenicallyprepared silicon dioxide according to the invention.

[0033] The pyrogenically prepared silicon dioxide according to theinvention as well as the dispersion according to the invention can beused for the preparation of glass bodies, for example, using the sol-gelmethod.

[0034] The pyrogenically prepared silicon dioxide according to theinvention, in the form of the aqueous dispersion, presents anadvantageous low viscosity.

EMBODIMENTS EXAMPLES Example 1

[0035] Preparation of a Dispersion

[0036] 245 g of distilled water are introduced into a glass beaker and,using an organic base, preferably TMAH (tetramethylammonium hydroxide),the pH is adjusted to 11.

[0037] Then, using a dissolver with a dissolver disk, 105 g of pyrogenicoxide are introduced into the water. The rpm of the dissolver isapproximately 1000 rpm. After the oxide has been completely worked intothe dispersion, the dispersion is subjected to a predispersion for 10-30min.

[0038] Then the dispersion is dispersed for 10-30 min using anUltra-Turrax rotor-stator dispersion apparatus at 10,000 rpm.

[0039] The viscosity of the dispersion is 40 mPas at a rotation speed of5 rpm.

Comparative Example 1

[0040] Preparation of a Dispersion

[0041] 245 g of distilled water are introduced into a glass beaker andthe pH is adjusted with an organic base, preferably TMAH(tetramethylammonium hydroxide) to 11.

[0042] Then, using a dissolver with a dissolver disk, 105 g ofconventional pyrogenic oxide are introduced into the water. The rpm ofthe dissolver is approximately 1000 rpm. After the oxide has beencompletely worked into the dispersion, the dispersion is subjected to adispersing action by means of a dissolver for 10-30 min.

[0043] Then, the dispersion is farther dispersed for 10-30 min with anUltra-Turrax rotor-stator dispersion apparatus at 10,000 rpm. Theviscosity of the dispersion is 240 mPas at a rotation speed of 5 rpm.

Example 2

[0044]17.2 g of the pyrogenic oxide are stirred with 27 mL of distilledwater and 2.57 mL of TMAH (tetramethylammonium hydroxide) to make ahomogeneous dispersion, as described in Example 1. After finishing thedispersion, 10 mL of acetic acid ethyl ester are added, and thedispersion is immediately poured into a mold.

[0045] After 12 min, the dispersion is gelled, and the gel body producedis removed after one hour from the mold and dried for 6 days at roomtemperature.

[0046] As a result of the drying, a microporous green body is produced.The green body is then sintered for four hours by zone sintering under avacuum at 1400° C. A sintered glass body without visible bubbles orpores is produced.

[0047] The silica according to the present invention are prepared asdescribed below.

Example A: (Comparative Example)

[0048] 600 kg/h SiCl₄ are vaporized at about 90° C. and are conveyed tothe central pipe of a conventional burner. Into this pipe there is thenintroduced 172 Nm³/h hydrogen as well as 245 Nm³/h air with an increasedoxygen content of 35 volume %. This gaseous mixture is then ignited andburned in the combustion chamber of a water cooled flame reactor. In acentral nozzle of a surrounding jacketed nozzle there is introduced 15Nm³/h hydrogen to reduce the tendency of forming a baked on coating. Inthe flame reactor there is additionally injected 290 Nm³/h air of normalcomposition.

[0049] After the cooling off of the reaction gases, the pyrogenic silicais separated from the hydrogen chloride containing gases in the filterunit. In a deacidification unit, the pyrogenic silica is treated for 20seconds dwell time at a temperature of 600 ° C. with steam and air,whereby the deacidification index DI of 6.2% by weight is obtained.

[0050] The deacidification index is defined as the relationship inweight units of the introduced steam to the deacidified pyrogenicsilica:

DI=(m H₂O/m SiO₂)×100%

[0051] where m=mass.

[0052] When this silica is formed into a dispersion according tocomparative example 1, there results a viscosity of 240 mPas in a 30%aqueous dispersion.

Example B

[0053] Preparation of a pyrogenic silica according to the invention witha low viscosity in a disperation.

[0054] 600 kg/h SiCl₄ are vaporized at about 90 ° C. and are conveyed inthe central pipe of a conventional burner. Into this pipe there isintroduced 172 Nm³/h hydrogen as well as 245 Nm³/h air with an increasedoxygen content of 35% by volume. This gaseous mixture is then ignitedand burned in the combustion chamber of a water cooled flame reactor. Inthe central nozzle of the surrounding jacketed nozzle there isintroduced 15 Nm³/h of hydrogen in order to avoid formation of baked oncoatings. Then in the flame reactor 290 Nm³/h normal air is injected..

[0055] After the cooling off of the reaction gases, the pyrogenic silicais separated from the hydrogen chloride containing gas in a filter unit.In a deacidification unit, the pyrogenic silica is treated for 20seconds dwell time at 600° C. with steam and air, whereby adeacidification index DI of 0.95% by weight if obtained.

[0056] When this silica is converted into a dispersion according toexample 1, there is obtained a viscosity value of 40 mPas in a 30%aqueous dispersion. TABLE 1 Conditions for the Preparation of PyrogenicSilica Primary air H₂ H₂ Viscosity SiCl₄ enriched Core Jacket BET D1 30%Nr. Kg/h Nm³/h Nm³/h Nm³/h M²/g [%] [mPas] A 600 245 172 15 50 6.2 240 B600 245 172 15 50 0.95  40

[0057] D1=Deacidification Index

[0058] Further variations and modifications of the foregoing will beapparent to those skilled in the art and are intended to be encompassedby the claims appended hereto.

[0059] EP priority applications 00 117 922.5 of Aug. 21, 2000 and 01 115613.0 of Jul. 3, 2001 are relied on and incorporated herein byreference.

We claim:
 1. A pyrogenically prepared silicon dioxide with the followingphysicochemical properties; a) average particle size (D₅₀ value) D₅₀≧150nm (dynamic light scattering, 30 wt %); b) viscosity (5 rpm, 30 wt %)η≦100 m·Pas; c) thixotropy of T_(i): (η(5 rpm))/(η(50 rpm))≦2; d) BETsurface area 30-60 m²/g; e) compacted bulk=100-160 g/L; and f) originalpH≦4.5.
 2. A pyrogenically produced silicon dioxide which has adeacidification index of less than 3% on a weight basis.
 3. Adispersion, containing a pyrogenically prepared silicon dioxideaccording to claim
 1. 4. Use of the pyrogenically prepared silicondioxide according to claim 1 for the preparation of glass bodies.
 5. Useof the dispersion according to claim 3 for the preparation of glassbodies.